1. Field of the Invention
This invention concerns a free machining steel for use in machine structures intended to be machined as components of industrial machines, automobiles and electric products and, more in particular, it intends to provide a free machining steel for use in machine structures having excellent machinability in a so-called Pb free steel, containing no substantial Pb as a machinability improving ingredient and also excellent mechanical characteristics.
2. Description of Related Art
Materials for components of industrial machines, automobiles and electric products are required to have good machinability since such components are manufactured by machining the materials. In view of the above, free machining steels for use in machine structures have usually been used as the materials and such free machining steels are often incorporated with Pb or S as a machinability improving ingredient and, particularly, it has been known that Pb provides excellent machinability with addition of a small amount.
As the technique described above, JP-A-205453/1984, for example, proposes a free machining steel for free machining low carbon sulfur steel in which all of Te, Pb and Bi are added in combination, MnS type inclusions each having a major diameter and a minor diameter of larger than a predetermined size and with a (major diameter/minor diameter) ratio of 5 or less are present by 50% or more of the entire MnS inclusions and the Al2O3 content in oxide inclusions is 15% or less.
Further, JP-A-23970/1987 proposes a technique of improving the machinability of a free machining low carbon sulfur-lead steel by a continuous casting method in which each of the contents for C, Mn, P, S, Pb, O, Si and Al is defined and the average size of MnS type inclusions and the ratio of sulfide type inclusions not bonded with oxides are defined thereby improving the machinability.
Each of the techniques described above concerns free machining steel with combined addition of Pb and S. As the problem of environmental pollution caused by Pb has been highlighted, use of Pb has tended to be restricted also in iron and steel materials and a study on the technique for improving the machinability in a so-called Pb free state has been progressed positively.
In view of the situation, a study for improving the machinability by controlling the form, for example, the size or the shape of sulfide type inclusions such as MnS has been predominant in the free machining sulfur steel, but no free machining steel that can provide machinability comparable with free machining Pb steel have yet been attained. Further, in the study of improving the machinability by controlling the form of the sulfide type inclusions, it has been pointed out also a problem that the sulfide inclusions such as MnS are deformed lengthwise along with plastic deformation of the base metal upon rolling or forging the steel material, which causes anisotropy in the mechanical characteristics and the impact resistance in a certain direction.
By the way, the machinability is evaluated by the items such as (1) cutting force, (2) tool life, (3) roughness on the finished surface and (4) chip disposability. Among the items, importance has been attached so far to the tool life and the roughness on the finished surface, but the chip disposability has also become an innegligible subject in view of operation efficiency and safety along with the recent automation or man-less trend in machining operation. That is, the chip disposability is a characteristic for evaluating disconnection of chips into shorter segments during machining. If the characteristic is worsened, chips extend spirally to bring about a trouble that they twine around the cutting tool to hinder the safety operation of machining. Existent Pb-added steels can provide a relatively good machinability also in view of the chip disposability but favorable characteristics have not yet been attained in the Pb-free steel materials.
This invention has been accomplished in view of the foregoing situations and intends to provide a free machining steel for use in machine structures that can stably and reliably provide, in a Pb-free state, excellent machinability (particularly, chip disposability and tool life) and mechanical characteristics (transverse direction toughness), which are comparable with those of existent Pb-added steels.
In accordance with this invention for attaining the foregoing object, there is provided a free machining steel for use in machine structures in which sulfide type inclusions are present wherein Mg is contained by from 0.0005 to 0.02 mass % and the distribution state for the sulfide type inclusions is controlled, to thereby improve mechanical characteristics. More specifically, there is provided a free machining steel for use in machine structures in which sulfide type inclusions are present, wherein Mg is contained by from 0.0005 to 0.02% (xe2x80x9c%xe2x80x9d means xe2x80x9cmass %xe2x80x9d here and hereinafter) and a distribution index F1 for the sulfide type inclusion particles defined by the following equation (1) is from 0.4 to 0.65:
F1=X1/(A/n)xc2xdxe2x80x83xe2x80x83(1), 
where
X1: represents an average value (xcexcm) obtained by actually measuring the distance between each of sulfide type inclusion particle in an observed visual field and other particle nearest thereto for all of particles present in the observed visual fields, measuring the distance for five visual fields and averaging them, where
A: represents an observed area (mm2), and
n: represents the number of sulfide type inclusions observed within the observed area (number).
Further, the foregoing object of this invention can be attained also by a free machining steel for use in machine structures in which Mg is contained by from 0.0005 to 0.02% and a distribution index F2 for the sulfide type inclusion particles defined by the following equation (2) is from 1 to 2.5:
F2="sgr"/X2xe2x80x83xe2x80x83(2), 
where
"sgr": represents a standard deviation for the number of sulfide type inclusion particles per unit area, and
X2: represents an average value for the number of inclusion particles per unit area.
In each of the free machining steels for use in machine structures, it is preferred to satisfy the condition that the ratio of a major diameter L1 to a minor diameter L2 (L1/L2) of the sulfide type inclusions is from 1.5 to 5, which can further improve the mechanical characteristic (transverse direction toughness) and the machinability (particularly, chip disposability and tool life).
The chemical ingredients of the free machining steel for use in the machine structures according to this invention preferably contains, in addition to Mg, C in an amount from 0.01 to 0.7%, Si in an amount from 0.01 to 2.5%, Mn in an amount from 0.1 to 3%, S in an amount from 0.01 to 0.2%, P in an amount 0.05% or less (inclusive 0%), Al in an amount of 0.1% or less (inclusive 0%) and N in an amount from 0.002 to 0.02%, respectively, in view of ensuring physical properties required as the free machining steel for use in machine structures. It is also useful to optionally incorporate at least one member selected from the group consisting of (a) Ti in an amount from 0.002 to 0.2%, Ca in an amount from 0.0005 to 0.02%, and from 0.0002 to 0.2% in total of rare earth elements and (b) Bi in an amount of 0.3% or less (exclusive 0%).
In order to solve the subjects described above, the present inventors have studied the relation, particularly, the relation between the chip disposability and the sulfide inclusions in the free machining steel with various points of view. As a result, it has been found that not only the size and the shape of the sulfide type inclusions such as MnS but also the distribution state of the sulfide type inclusions has a close concern with the chip disposability. As a result of a further study, it has been found that a free machining steel for use in machine structures having, in the Pb-free state, excellent mechanical characteristics (transverse direction toughness) and chip disposability, and also excellent tool life can be provided by controlling the distribution state of the sulfide type inclusions and incorporating Mg in an amount from 0.0005 to 0.02%, and the present invention has been accomplished. The function and the effect of the invention are to be explained below.
The free machining steel for use in machine structures of excellent mechanical characteristics according to this invention has features in incorporating Mg in an amount from 0.0005 to 0.02%b and in controlling the distribution state of the sulfide type inclusions as described above.
Mg: 0.0005xcx9c0.2%
When Mg is added to a free machining steel, Mg-containing oxides form a nucleus for sulfide type inclusions to control the form of the inclusions and decrease large sulfide type inclusions thereby capable of obtaining a free machining steel for use in machine structures excellent both in the mechanical characteristics (transverse direction toughness) and the chip disposability. Further, when Mg is added, an oxide composition which is usually present as a hard alumina type oxide is transformed into an Mg-containing oxide to lower the hardness of the hard alumina type oxide. The disadvantage which may be caused by the hard Mg-containing oxide can be mitigated by the effect that the Mg-containing oxide is surrounded with the sulfide leading to the improvement for the tool life. However, if the Mg content is less than 0.0005%, the solid solubilized amount of Mg in the sulfide is not sufficient and the form of the sulfide type inclusions can not be controlled effectively. Further, if it exceeds 0.02%, the sulfides are excessively hard to lower the machinability (chip disposability).
As has been described above, disconnection of the chips into fine segments is required, as one of the evaluation items for the machinability in the automated machining. The present inventors have confirmed that disconnection of the chips is caused by the occurrence of cracks due to stress concentration to the vicinity of the inclusions present in the steel. Further, when inclusions are present being extended lengthwise in the steel a favorable chip disposability can be obtained in the machining along a certain direction but the chip disposability is lowered abruptly when the machining direction changes. On the other hand, in the case of spherical inclusions, although there is no anisotropy that the machinability changes depending on the machining direction, the chip disposability is not always satisfactory.
When the present inventors have made various studies on the means for evaluating the distribution state of the sulfide type inclusion particles based on the analysis during machining as described above, it has been found that the foregoing object can be attained effectively when Mg is incorporated by 0.0005 to 0.02% and the distribution index F1 or F2 for the sulfide type inclusion particles defined by equation (1) or (2) above is within a predetermined range. Then, the distribution indexes F1, F2 of the sulfide type inclusion particles are to be explained.
At first, the distribution index F1 for the sulfide type inclusion particles means the value for the ratio: [(X1/(A/n)xc2xd], in which X1 represents an average value obtained by actually measuring a distance between each of sulfide type inclusion particles and other particle nearest thereto in an observed visual field, for all of the particles present in the observed visual field, measuring the distance with respect to five visual fields and averaging them, and (A/n)xc2xd means an inter particle distance when all of the observed particles are arranged uniformly on lattice points (where A represents an observed area (mm2) and n represents the number of sulfide type inclusion particles observed within the observed area (N).
As an example, explanation is to be made to a case where the twelve sulfide type inclusion particles are present in the observed visual field with reference to FIG. 1. In the actual observation visual field, sulfide type inclusion particles are distributed as shown in FIG. 1A and, assuming the nearest distance on each of the sulfide type inclusions as x1-x12, the average value X1 is represented as:
X1=(x1+x2+ . . . x12)/12 
Assuming that the sulfide type inclusion particles are distributed uniformly as shown in FIG. 1B, the nearest distance on each of the sulfide type inclusion particles is represented as:
x1=x2 . . . =x12 
Assuming the observed area as A, the nearest distance X2 can be represented as:
X2=(x1+x2+ . . . x12)/12=(A/12)xc2xd
The X1 to X2 ratio is defined as the distribution index F1 for the sulfide type inclusion particles.
The distribution index F1 for the sulfide type inclusion particles defined as described above takes a value approximate to 1 when the sulfide distribution is completely uniform but deviates from 1 and takes a value less than 1 when the distribution is not uniform. Then, according to the study of the present inventors, in the free machining steel according to this invention containing from 0.0005 to 0.02% of Mg, the form and the balance of the distribution state of the sulfide type inclusion particles are improved and both the chip disposability and the transverse direction toughness are favorable when the value F1 is within a range from 0.4 to 0.65. On the other hand, if the value exceeds 0.65, although the sulfide type inclusion particles are present uniformly, the chip disposability can not be said favorable. Further, if the value F1 is less than 0.4, the sulfide type inclusion particles are agglomerated and extended lengthwise during rolling or forging, failing to obtain a free machining steel excellent in both of the characteristics of the chip disposability and the transverse direction toughness.
On the other hand, the distribution index F2 for the sulfide type inclusion particles means a value obtained by dividing a visual field of a certain area into lattice, and normalizing the standard deviation "sgr" for the number of sulfide type inclusions present in each of unit lattices by an average value X2 for the number of sulfide type inclusion particles per unit area. In this case, when the sulfide type inclusions are distributed completely uniformly, the value F2 approaches 0. Then, in the free machining steel according to this invention containing Mg from 0.0005 to 0.02% of Mg, it has been found that when the value F2 is within a range from 1 to 2.5, the form and the distribution state of the sulfide type inclusion particles are favorable and both of the chip disposability and the lateral direction toughness are satisfactory. On the other hand, if it is less than 1, the sulfide type inclusion particles are distributed uniformly to deteriorate the chip disposability. Further, when the value F2 exceeds 2.5, the sulfide type inclusion particles are agglomerated and extended lengthwise by rolling or forging failing to obtain satisfactory transverse direction toughness.
Further, in the free machining steel for use in machine structures according to this invention, the ratio of the major diameter L1 to the minor diameter L2 (L1/L2 aspect ratio) for the sulfide type inclusions is preferably controlled to 1.5-5, which can provide further excellent chip disposability and transverse direction toughness. That is, the sulfide type inclusions are deformed to some extent by rolling or forging. When the aspect ratio for the sulfide type inclusions is less than 1.5 in average upon cutting the specimen in parallel and observed, the chip disposability is deteriorated. On the other hand, if the value is too large and exceeds 5, the transverse direction toughness is lowered.
There is no particular restriction on the kind of the steel material but with a view point of satisfying the characteristics required as the free machining steel for use in mechanical structure, it is preferred to incorporate, in addition to Mg, C in an amount from 0.01 to 0.7%, Si in an amount from 0.01 to 2.5%, Mn in an amount from 0.1 to 3%, S in an amount from 0.01 to 0.2%, P in an amount of 0.05% or less (inclusive 0%), Al in an amount of 0.1% or less (inclusive 0%) and N in an amount from 0.002 to 0.02%, respectively. When the compositional chemical ingredients are controlled as described above, good characteristics can be obtained while retaining required tensile strength as the free machining steel for use in machine structures as-the free machining steel for use in mechanical structure, and the distribution and the shape of the sulfide type inclusions are also improved to make both the machinability and the mechanical characteristics more excellent. The function for each of the ingredients described above is as shown below.
C: 0.01xcx9c0.7%
C is a most important element for ensuring the strength of a final product and the C content is preferably 0.01% or more, with a view point described above. However, if the C content becomes excessive, since the toughness is deteriorated and it gives undesired effect also on the machinability such as the tool life, it is preferably 0.7% or less. Further, a more preferred lower limit for the C content is 0.05% and, more preferable, upper limit is 0.5%.
Si: 0.01xcx9c2.5%
Si is effective as a deoxidation element and in addition also contributes to the improvement of strength of mechanical structural components by solid solution strengthening. In order to attain such an effect, it is contained, preferably, by 0.01% and, more preferably, by 0.1% or more. However, since excessive content gives an undesired effect on the machinability it is, preferably, 2.5% or less and, more preferably, 2% or less.
Mn: 0.1xcx9c3%
Mn is an element not only contributing to the improvement hardenability of a steel material to increase the strength but also contributing to the formation of sulfide type inclusions to contribute to the improvement of the chip disposability. For effectively attaining the effect, it is incorporated, preferably, by 0.1% or more. However, since excessive content rather deteriorates the machinability it is, preferably, 3% or less and, more preferably, 2% or less.
S: 0.01-0.2%
S is an element effective to the formation of sulfide type inclusions for improving the machinability. For attaining the effect, it is contained by, preferably, 0.01% or more and, more preferably, 0.03% or more. However, since excess S content tends to cause cracks starting from sulfides such as MnS it is, preferably, 0.2% or less and, more preferably, 0.12% or less.
P: 0.05% or less (inclusive 0%)
Since P tends to cause grain boundary segregation to deteriorate the impact strength, it should be kept to 0.05% or less and, more preferably, 0.02% or less.
Al: 0.1% or less (inclusive 0%)
Al is important as a deoxidation element upon making steel material by melting and, in addition, effective for forming nitrides for the refinement of the austenitic crystal grains. However, since excess content rather makes the crystal grain coarser to give an undesired effect on the toughness it is kept, preferably, to 0.1% or less and, more preferably, to 0.05% or less.
N: 0.002xcx9c0.02%
N forms, together with Al or Ti, fine nitrides to contribute to the improvement for refinement and increase in the strength of the texture. In order to attain the effect, it is incorporated by 0.002% or more. However, since excess content may possibly cause large nitrides it should be kept to 0.02% or less.
Preferred compositional chemical ingredients in the free machining steel for use in machine structures according to this invention are as has been described above, and the balance basically comprises iron and inevitable impurities. Since this invention has a technical feature in defining the distribution state of the sulfide type inclusions in the free machining steel containing Mg in an amount from 0.0005 to 0.02% as described above, other compositional chemical ingredients than Mg do not restrict the invention but the composition may be deviated somewhat from the preferred chemical ingredient composition described above depending on the application uses and the required characteristics for the free machining steel for use in machine structures. Further, in addition to the, the following elements may optionally be incorporated effectively.
One or More of Elements Selected from the Group Consisting of: Ti: 0.002xcx9c0.2%. Ca: 0.0005xcx9c0.02% and Rare Earth Element: 0.0002xcx9c0.2% in Total
When the steel material is made by melting, the distribution state of the sulfide type inclusion particles changes by the addition of Ti, Ca, or rare earth element and more excellent characteristics can be obtained compared with the case of not adding them. However, if the Ti content is less than 0.002%, the addition effect is insufficient. On the other hand, if it is contained excessively beyond 0.2%, the impact resistance is remarkably deteriorated. Further, in a case of Ca, the addition effect is insufficient if the content is less than 0.0005%, whereas excessive addition amount of 0.02% or more causes lowering of the impact resistance like that for Ti. Further, in a case of rare earth element such as Ce, La, Pr or Nd, the additive effect thereof is not sufficient if the content is less than 0.002% in total, whereas the impact resistance is lowered like that for Ti or Ca if the content exceeds 0.2%. The elements such as Ti, Ca or rare earth element may be added either alone or two or more kinds of them may be added simultaneously. Since the transverse direction toughness is deteriorated if the total content exceeds 0.22%, the upper limit is defined as 0.22%.
Bi: 0.3% or less (exclusive 0%)
Bi is an element effective to the improvement of the machinability but excess content not only saturates the effect thereof but also deteriorates the hot forgeability to lower the mechanical characteristics, so that it should be 0.3% or less.
Further, in addition to Ti, Ca and the rare earth elements described above, Ni, Cr, Mo, Cu, V, Nb, Zr or B may also be incorporated to obtain a free machining steel for use in machine structures capable of satisfying the conditions of this invention.
When the melting method is used as a method of manufacturing the free machining steel for use in machine structures according to this invention, it is important to select the kind of Mg alloys used for the addition of Mg, and control the dissolved amount of oxygen upon adding the Mg alloy, the time from the addition of the Mg alloy to the start of casting, and the mean solidification rate (cooling rate) after the start of the casting to solidification in a well balanced manner. By controlling them in a good balance, it is possible to incorporate Mg by 0.0005-0.02% and control the distribution indexes F1, F2 for the sulfide inclusion particles defined by the formula (1) or (2) within the range of the invention. Particularly, the dissolved amount of oxygen upon addition of the Mg alloy is important for providing the effect of the Mg and the dissolved amount of oxygen is adjusted by optionally controlling the Al addition amount before addition of the Mg alloy in the examples to be described later. Further, there is no particular restriction on the kind of the sulfide type inclusions as an object of the invention and they may be sulfides of Mn, Ca, Zr, Ti, Mg and other elements, composite sulfides thereof, carbon sulfides or acid sulfides, so long as the distribution state of the inclusions can satisfy the conditions as defined in equation (1) or (2).